Yeah I think I've put like $800 total into my machine and I'm still on socket A
Granted it's because I've slowly had to acquire 'high performance' parts and when I did my case mod I put like $100 into cooling alone, but my system still needs one more part, a graphics card to be what I consider 'perfect'
Point is $500 is a good amount to build a fairly decent budget machine, but games like doom3 are very taxing and you'd need some good parts to go along with it for it to play smoothly.
I'd suggest getting a gig of RAM high performance or not, doesn't really matter IMO....if you are overclocking high performance is worth it, other than that you're fine with valueRAM
Then you'd need a decent Graphics card which currently your best bet would be to try and save for the 6600GT atleast which is $220
A good strong PSU, because generally the ones that come with cases are crap. That's why I got a thermaltake 560w PSU, which alone was $100 but it's worth it because I've never even come close to taxing it to the point it can't handle.
And of course the CPU.....best bet unless you wanted to get into overclocking would be to get the best AMD64 mobo and CPU you could afford but that's easily $300+ right there.
So like gaara said you should try and save up for like $750 and I think you could get a much better machine